Planar magnetic structures, such as transformers, offer many advantages over traditional magnetic devices. These advantages include less weight, lower profiles, smaller footprints, design flexibility and greater efficiency.
International safety standards set many of the parameters for the design of these devices. The spacing distance between primary and higher-order windings required to withstand a given working voltage is specified in terms of creepage and clearance. “Creepage” is defined as the shortest distance between two electrically active parts as measured along an insulative path. “Clearance” which is defined as the shortest distance between two electrically active parts as measured in air, must be, for instance, at least 4 mm for operating voltages of less than 250V. Additionally, the thickness of the sheets of dielectric used as spacers between the windings must be at least 0.4 mm.
A popular method of assembling planar magnetic devices uses thin, stamped metal windings interleaved with thin spacers of dielectric material for isolation. These metal windings are single-turn due to the extreme flexibility of the thin metal when they are fashioned with many turns. This flexibility adversely affects both the alignment of the winding and the manufacturability of the assembly. In instances where there is a need for a large number of turns in a winding, either several of the single-turn windings are connected together, thickening the stack-up, or a substrate with a metal film patterned in a multiple-winding configuration is used.
Another disadvantage in the current art is the use of a thick centrally placed dielectric bobbin, which acts as a holder for the interleaved layers while providing enhanced isolation between the primary and secondary windings by completely encompassing the primary winding, thereby addressing the creepage and clearance specifications for these devices.
The use of the bobbin is disadvantageous in two ways. First, leakage inductance for these assemblies is relatively high because its value depends largely on the thickness of the insulating material between the primary and secondary windings of a magnetic device, and the bobbin is much thicker than the thin dielectric spacers used for interleaving with the windings outside of the bobbin. Second, despite the high surface-to-volume ratio of these devices which normally would allow for a large heat removal capacity, removing heat from that portion of the assembly which is surrounded by the thick bobbin is difficult. These problems are compounded when a thick substrate is employed for the primary winding in devices which require a many-turned winding.
Yet another method of assembling these devices bypasses the bobbin and uses an over molding process to fully encapsulate the assembly. The layers are placed into a carrier positioned at the bottom of the stack, with spacers provided to maintain relatively large air gaps between the planar metal windings and dielectric spacers to allow the mold compound to fully penetrate between the interleaved layers. The resulting assembly does not have creepage and clearance issues, but the over molding compound greatly increases the leakage inductance and makes heat removal problematic. Cracking of the mold compound during thermal cycling is also a concern with this type of assembly.
Therefore, an object of the present invention is to provide a planar magnetic device that can meet clearance and creepage requirements without the use of either a substrate or thick central bobbin while minimizing the parasitic inductance between the primary and secondary windings and facilitating the removal of heat from the assembly.
Another object of this invention is to provide a planar magnetic device which can provide for the use of planar metal windings with more than one turn without employing the use of a substrate.
Still another object of this invention is to provide a method of assembling such a planar magnetic device.